Saharan dust is expected to settle over parts of the UK today as forecasters predicted temperatures are set to remain unseasonably warm in the lead-up to Christmas.
Officials have issued health warnings about reduced air quality, saying the elderly, asthma sufferers and those with breathing difficulties were most at risk.
The Department for Environment, Food and Rural Affairs (Defra) said high levels of pollution were expected over southern and central England, with Lincolnshire bearing the brunt.
Defra said: "There is a risk of moderate or high levels of air pollution across parts of England and Wales on Thursday due to Saharan dust, although large uncertainties remain with the timing and amount of dust. Elsewhere, levels of air pollution are expected to be low."
The Met Office said the mild southerly airflow currently affecting the UK, leading to the fourth mildest start to December since 1960, has increased the chance of seeing some of the dust.
MeteoGroup said the tropical air coming off the Atlantic from the South West could see above-average temperatures continue into next week.
Forecaster George Goodfellow said the mercury could rise as high as 17C in parts of south-east England.
He said: "In terms of the warm weather, that's sticking around. We are looking at temperatures perhaps up to 17C, even if you look as far ahead as Saturday. This is mainly in eastern areas, like the South East and East Anglia.
"Christmas may be a bit too early to see but looking into next week there are still suggestions of temperatures between 11C and 15C as far ahead as next Tuesday. Even towards the end of next week the general gist is, with the south westerly winds, it will stay relatively mild."
Saharan dust is lifted from its source in west Africa by strong winds and is blown northwards in weather systems heading towards the UK.
It is lifted by strong winds and can reach very high altitudes. From there, it can be transported worldwide by winds, covering distances of thousands of miles before getting caught in rain droplets in clouds, falling to the ground in rain. When the water evaporates, a thin layer of dust is left on surfaces, such as cars.
Dr Sotiris Vardoulakis, head of Public Health England's Environmental Change Department, said: "While most people will not be affected by short term peaks in ambient air pollution, some individuals, such as those with existing heart or lung conditions, may experience increased symptoms.
"On occasions where levels are high, adults and children with lung problems, and adults with heart problems, should reduce strenuous physical exertion, particularly outdoors, and particularly if they experience symptoms.
"People with asthma may find they need to use their reliever inhaler more often. Older people should also reduce physical exertion. Anyone experiencing discomfort such as sore eyes, cough or sore throat should consider reducing activity, particularly outdoors."